Conventional computer programs comprise a list of instructions which are sequentially executed to manipulate data being processed. It is a time consuming, skilled and expensive task to write computer programs. In order to help reduce these problems, techniques have been developed which seek to assist in enabling the reuse of code between different computer programs. Such object oriented programming techniques divide the code and the data manipulated by the code into portions called objects. The encapsulation of both the code and data makes the objects easier to reuse in differing programs. Execution of such object oriented programs involves the sending of messages between these objects. The objects are organized into hierarchies in which child objects inherit function from their parents. This also reduces the need to write more than one piece of code to perform any given function. One or more objects from one program can be transferred for use in another program.
There are a number of different object oriented programming languages such as Smalltalk and C++ (Smalltalk is a trademark of Digitalk). These various object oriented programming languages have different properties making them suitable for varying purposes. Smalltalk is an interpreted language making it suitable for prototyping as it is easy to modify, but unsuitable as a production language as it is relatively slow in operation. Conversely, C++ is a compiled language making it unsuitable for prototyping as it is difficult to alter, but suitable as a production language as it is fast in operation. All computer languages have both advantages and disadvantages; no one computer language is the best for all purposes. It would be desirable if different parts of a computer program could be written in different languages to better match the circumstances of each portion of code with the language it uses.
With conventional non-object oriented computer programs the task of allowing a computer program in a first language to a cooperatively process with a computer program in a second language would be a complex matter and would require a significant additional overhead. In contrast, object oriented computer programs have a granular or encapsulated structure in which the objects appear to be inherently isolated and independent of one another. The objects normally interact only through the exchange of messages. In theory, this feature should make cooperative inter-language processing easier.
However, in practice, the different object oriented programming languages operate in quite different ways. A message suitable for receipt by an object in one language environment cannot be understood by an object in another language environment. The problem is not one of merely translating the syntax of the message. The way different languages manage the passing of messages between objects vary considerably. One way to enable a first computer program to send a message to a second computer program would require the first computer program to go inside the second computer program to find the information it needs to generate the message. Providing a computer program in a first language capable of going inside a computer program written in another language in search of this information would be a problem of considerable magnitude, and with an accompanying significant overhead. Thus, whilst at first sight the use of object oriented programming techniques would seem to make communication between objects in different languages merely a matter of communicating a message, in practice this is a considerable oversimplification and the difficulties are substantial.